The present invention relates to an elevator and, more particularly, to a control system for controlling the movement of an elevator car.
In order to obtain a comfortable ride on the elevator car during the movement thereof, it is preferable to gradually increase the speed of the elevator car at the beginning of the movement of the car until it reaches a predetermined speed and, to gradually decrease the speed to stop the movement. More particularly, during the increase in speed of the elevator car, that is, during the acceleration thereof, it is preferable to gradually increase the acceleration. When starting the movement of the elevator car, it is accelerated gradually to reach a predetermined acceleration. Thereafter, the acceleration of the elevator car is gradually decreased to zero to run the elevator car at a predetermined speed. In a similar manner, during the decrease in speed of the elevator car, that is, during the deceleration, it is preferable to gradually increase the deceleration at the beginning thereof to reach a predetermined deceleration and to gradually decrease the deceleration at the ending thereof to stop the elevator car.
In order to accomplish the above described movement of the elevator car, there have been proposed various methods and systems for controlling the driving means for driving the elevator. In FIG. 1, there is shown a circuit diagram of one conventional control system for controlling the movement of the elevator car. This control system includes two induction motors HM and LM whose rotating shafts are connected to each other and are further connected to a traction sheave (not shown) which moves the elevator car up and down upon rotation thereof. The induction motor HM or high speed motor is provided for starting and accelerating the elevator car while the other induction motor LM or low speed motor is provided for decelerating the elevator car. The high speed motor HM is connected to a three phase AC power source through three lead lines R, S and T each including a plurality of resistors connected in series. The movement of the elevator car is described hereinbelow with reference to the graph of FIG. 2 showing the relation between the speed of the elevator car and the time.
When the three phase AC power is supplied to the induction motor HM through the resistors, the induction motor starts to rotate and, thus the elevator car starts moving (region A in FIG. 2). As the resistors in each line are shortcircuited one after another by a suitable switching means, the rotation of the induction motor HM increases to accelerate the elevator car and to cause the elevator car to move at a predetermined speed (region B in FIG. 2). When the elevator car reaches a point a predetermined distance away from the point where the car should stop, the induction motor HM is disconnected from the power source and the other induction motor LM is connected to the power source so as to decelerate the elevator car by the regenerative braking effect produced by the low speed induction motor LM (region C in FIG. 2). However, this regenerative braking does not completely stop the elevator car but only reduces the speed of the car to a very low speed determined by the rated revolution thereof. Then, when the elevator car reaches the point where the car should stop, the induction motor LM is disconnected from the power source and electromagnetic braking is applied to the elevator to completely stop the elevator car (region D in FIG. 2).
Therefore, the control system described above is disadvantageous because the elevator car is accelerated to a greater degree each time one of the resistors is shortcircuited and because it takes a very long period of time before the car is completely stopped from the moment when the car is decelerated. Furthermore, the movement of the car changes abruptly at the moment when the induction motor HM starts to move the car or when the induction motor LM starts to decelerate the car. Therefore, this gives an uncomfortable ride to the passengers in the elevator car.